1. Field of the Invention
The invention relates to doping of a semiconductor device, and more particularly to controlling dopant concentration of a semiconductor device in accordance with gate length.
2. Background Description
Doping concentration of the active region of a semiconductor device affects many performance characteristics of the semiconductor device. Additionally, the gate length of the active region of a semiconductor device also affects many characteristics of the semiconductor device. For example, for a given doping concentration, as the gate length of the semiconductor device becomes smaller, the semiconductor device becomes more and more susceptible to short channel effects such as punch through and high leakage current. “Punch through” of a semiconductor device means that as the gate length of a semiconductor device becomes smaller, there is a greater tendency for current to flow between the source and drain of the semiconductor device which cannot be controlled by the gate. Accordingly, under certain circumstances, a semiconductor device which experiences punch through will tend to conduct regardless of the voltage applied to the gate. Such a device will also tend to have a high leakage current. A semiconductor chip employing such devices will have high off-state power. Thus, as the semiconductor device becomes smaller, punch through and other short channel effects increase the leakage current of the device and power of the chip.
One method to reduce short channel effects as gate length decreases includes increasing the doping concentration in the active region of the semiconductor device. Accordingly, semiconductor devices with shorter gate lengths benefit from having a higher doping concentration between the source and the drain thereby mitigating short channel effects.
However, higher doping concentration in the active regions of a semiconductor device increases the semiconductor device's threshold voltage. Thus, as a given semiconductor device's doping concentration in the active region is increased to mitigate short channel effects, there is a corresponding increase in the threshold voltage of the semiconductor device. This in turn reduces the drive current of the semiconductor device and reduces the performance of the chip.
Devices with longer gate lengths suffer less from short channel effects than devices with shorter gate lengths, and longer gate devices do not need to have as high doping concentrations in the active region as shorter gate devices. Devices with a long gate channel preferably have a lower doping concentration in the active region relative to the preferred doping concentration in the active region of a short gate device because higher doping concentrations reduce the drive current of a device. Accordingly, devices having short gate lengths benefit from having higher doping concentrations in the active region, and devices with longer gate lengths benefit from having lower doping concentrations in the active region.
Because of process variations, a semiconductor chip or wafer will have multiple semiconductor devices having various gate lengths. In order to maximize the performance/power ratio of the total chip, it is preferable to maximize the ratio of doping concentration in the short channel devices to doping concentration in the long channel devices. Accordingly, the leakage current and off-state power of the short channel devices will be reduced and the drive current and performance of the long channel devices will be increased, thereby increasing the performance of the chip for a given power.
The channel of a semiconductor device can be doped with two different types of dopants, i.e. an acceptor type dopant or a donor type dopant. The concentration of an acceptor type dopant is denoted by NA and a donor type dopant by ND. For an NFET, net doping concentration in the channel is defined as the acceptor type dopant concentration minus the donor type dopant concentration, i.e. NA−ND, whereas for a PFET, it is defined by ND−NA. For an NFET, the net doping concentration in the channel should be acceptor type (NA>ND) to control the short channel effects, while for a PFET, the reverse is true. A donor type dopant in an NFET channel is also referred to as a compensating dopant, whereas in a PFET channel, an acceptor type dopant is referred to as a compensating dopant.
In order to maximize the performance/power ratio of a chip, the net doping concentration in the channel should be as high as possible in short channel devices and as low as possible in long channel devices. For an NFET, one way to achieve this is by making NA high in short channel devices and low in long channel devices. This can be achieved via. a halo implant which is well known in the art.